1. Field of the Invention
This invention relates to optical scanning methods and apparatus, and more particularly, to an optical scanning method and apparatus which eliminates the effect of deviations in the angle between a mirror and the rotational axis of the mirror. In optical scanning devices utilizing polygon mirrors, the error eliminated by the present invention is referred to as the facet to axis error.
2. Description of the Prior Art
Mirror to rotational axis error presents serious problems in the design of precision optical scanning systems. In optical scanners utilizing polygon mirrors, the mirror facets typically are ground with a facet to rotational axis error on the order of plus or minus thirty seconds. Up to the present time, it has been possible to achieve facet to axis error tolerances of on the order of plus or minus five seconds, but achieving such close tolerances is costly, time consuming and requires an extremely high level of skill. In some optical scanner applications, it is desirable to virtually eliminate the effects of the facet to axis error of the polygon mirror so that the scanned optical output signal retraces precisely the same path during each scan.
U.S. Pat. No. 3,897,132 (Meeussen) discloses a facet to axis error correction system for a rotating polygon mirror which utilizes a corner reflector mirror in combination with a positive lens. Column 4, lines 18-21 of this patent discloses that reflective prisms may be substituted for the reflective plane mirror surfaces. Because of the relative placement of the polygon mirror, the corner reflector mirror and the postive lens, the Meeussen optical scanning device generates a curved, scanned optical output beam.
Another device for eliminating or minimizing the facet to axis error in a polygon optical scanner is disclosed in U.S. Pat. No. 4,054,361 (Noguchi). In this device a beam of collimated light is passed through or reflected by a single optical element a first time to form a line image on the rotating polygon mirror. The reflected beam of light from the polygon mirror facet is passed through or reflected by the optical element a second time and the resulting collimated beam is passed through the image forming optical system to form a light spot on the image plane. As the polygon mirror is rotated, the light spot successively scans the image plane without any displacements of the scanning line resulting from facet to axis error.
Another system for eliminating the effects of facet to axis error is disclosed in U.S. Pat. No. 4,054,360 (Osaka). This device utilizes a polygon mirror in combination with three lenses and a planar mirror to generate a scanned optical output signal free of the effects of facet to axis errors.
U.S. Pat. Nos. 3,762,793 (Ullstig), 3,750,189 (Fleischer), 3,865,465 (Tatuoka) and 3,995,110 (Starkweather) disclose various types of optical scanning systems which utilize a first cylindrical lens positioned between a light source and a rotating polygon mirror and a second cylindrical lens positioned between the polygon mirror and a focusing lens to correct for the mirror facet to axis errors.
Another prior art system measures the facet to axis error of each mirror facet of a polygon mirror. The reflected output beam from the polygon mirror is passed through an acoustic modulator which is programmed to correct the deviation of the beam from each mirror facet by an amount precisely equal to the facet to axis error of each facet of the polygon mirror. This system is only capable of correcting for static errors and cannot correct dynamic facet to axis errors caused by polygon mirror support bearing deflections or thermally generated errors.
In another related prior art system, a feedback compensation system detects errors in the beam of light reflected from the leading edge of each facet of a rotating polygon mirror. A correction signal is generated which controls an acoustic modulator to compensate for non-repeatable, dynamic errors. This prior art feedback compensation system is not capable of correcting for facet to axis errors which take place during the scan of a mirror facet and therefore cannot completely eliminate facet to axis errors.